Uptime and SLA Calculator
See how much downtime an availability target allows per day, week, month, quarter and year, and what a chain of dependencies adds up to.
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Input
Turn an uptime percentage into allowed downtime
Output
99.9% allows 43m 50s a month
| Period | Allowed downtime |
|---|---|
| Day | 1m 26s |
| Week | 10m 5s |
| Month (30.44 days) | 43m 50s |
| Quarter | 2h 11m 29s |
| Year (365.25 days) | 8h 45m 58s |
| Composite availability | 99.8401% |
|---|---|
| Composite downtime per month | 1h 10m 6s |
| Meets the target | no |
Components in series multiply: three services at 99.9% each give about 99.7% together, which is why a 99.99% SLA needs redundancy rather than better servers. Redundant copies in parallel combine as 1 - (1 - a)(1 - b). Check what the provider counts as downtime; most exclude maintenance windows and partial outages.
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About this tool
Method, assumptions, practical next steps, and limits.
The nines
99 percent allows about 7 hours 18 minutes of downtime a month, 99.9 about 44 minutes, 99.99 about 4 minutes and 99.999 about 26 seconds. Each extra nine costs roughly ten times more in redundancy and process, so pick the target from what an outage costs rather than from a round number. The Site Reliability Engineering book's chapter on risk frames it as an error budget to spend.
Dependencies multiply
A request that needs a load balancer, an app and a database, each at 99.9 percent, succeeds about 99.7 percent of the time. Redundancy works the other way: two independent copies at 99 percent give 99.99. That is why high targets come from removing single points of failure, not from better hardware.